A spark plug is attached to a combustion apparatus of an internal combustion engine or the like, and is employed for ignition of an air-fuel mixture in a combustion chamber. Generally, a spark plug includes an insulator having an axial hole extending in the direction of an axial line; a center electrode inserted into a forward portion of the axial hole; a metallic shell provided around the insulator; and a ground electrode which is provided at the forward end of the metallic shell and which provides a gap in combination with the center electrode. The metallic shell has, on an inner wall thereof, an annular protrusion which projects inwardly in a radial direction and whose center coincides with the axial line. The insulator is inserted in the metallic shell and crimped thereto by means of a rear end portion of the metallic shell bended through application of a load to the rear end portion, such that an engagement portion provided at the forward end of the insulator seats on a diameter-decreasing portion (i.e., a rear side surface) of the protrusion. An annular plate packing is provided between the engagement portion and the diameter-decreasing portion for the purpose of improving the gas-tightness therebetween (see, for example, Japanese Patent Application Laid-Open (kokai) No. H10-289777).